Mechanisms for Phase Separation

The thermodynamic model presented above only predicts when phase separation will occur. There are, however, two mechanisms by which phase separation can actually occur. The first mechanism is similar to that discussed in an earlier chapter for precipitation of crystals from a melt, where a nucleus is formed and then grows with time. By analogy, this mechanism is termed nucleation and growth. Many of the same factors which control crystal formation also affect phase separation by this mechanism. The second mechanism is termed spinodal decomposition. This mechanism involves a gradual change in composition of the two phases until they reach the immiscibility boundary. 

Figrue 4.2 Idealized immiscibility region in a binary system 

---

While the evidence for dual phase continuity provided by Figure 5 does not indicate directly any mechanism for phase separation, or the shape of the phases, dispersed, spherical polystyrene domains probably would not yield results of this type. By hind sight, the data are consistent with the notion of spinodal decomposition and cylindrical domains. 

In the same manner, with decreasing of diffusion coefficient and interaction parameter, the spinodal is reached during the evolution of the system in the pregel stage. The very low values of interfacial tension in rubber modified epoxies (interfacial tension of polymer-polymer-solvent system were reported in range of 10-4-10-1 mN/m) therefore lead to an NG mechanism for phase separation. 

The mechanisms for phase separation from the metastable and the unstable regions is very different. In the unstable r on... 

Two mechanisms for chiral separations using chiral mobile-phase additives, analogous to models developed for ion-pair chromatography, have been... 

To the acidic distillate in the 125-mL separatory funnel, add 5 mL of 50% sodium hydroxide and 15 mL of dichloromethane. Cap the separatory funnel tightly, and allow its contents to cool for 30 min. Heat created by the addition of caustic to the acidic distillate will cause some of the dichloromethane to volatilize, creating pressure in the funnel therefore, the cap must be secured tightly to the funnel. Escaping solvent will result in loss of analytes. Shake the funnel for 5 min on a mechanical shaker. Allow 15 min for phase separation after shaking the funnel. Drain the lower dichloromethane layer into a second 125-mL separatory funnel. Extract the aqueous layer a second time with 15 mL of dichloromethane. Following shaking of the funnel and phase separation, combine both dichloromethane layers in the same 125-mL separatory funnel. 

Convincing evidence for phase separation was obtained from the photopolymerization behavior of 6 in the mixed 6/DSPE monolayer films. Photopolymerization of diacetylenes is a topotactic process which requires the proper alignment of the 1,3-diyne moieties [35]. Thus diacetylenes typically polymerize rapidly in the solid state but not in solution. Polymerization is triggered by ultraviolet irradiation and proceeds via a 1,4-addition mechanism yielding a conjugated ene-yne backbone (Fig. 5). The reaction can be followed by the growth of the visible absorption band of the polymer. 

This proposed mechanism for protein separations is supported by the recent theoretical studies of Horvath ef al. (29) and Horvath and Melander (28). In these studies, the hydrophobic effect in aqueous-organic systems (termed the solvophobic theory) was used to predict the retention of peptides on a nonpolar column. These authors found that the dominant interactions were between the mobile and stationary phases and between the mobile phase and the sample molecules. The driving force in both interactions was the shielding of a nonpolar region of either the column or sample molecule from the polar aqueous phase. 

Ultrathin porous glass membranes with variable texture properties were prepared from a Si02-rich sodium borosilicate initial glass by careful fine timing of the conditions of heat treatment for phase-separation. Pore sizes between < 1 and 120 nm can be realized. The membranes are characterized by a narrow pore size distribution. The transport, optical and mechanical properties vary with the pore size. The tailorable texture and transport characteris-... 

The Blume-Emery-Griffiths (BEG) model is one of the well-known spin lattice models in equilibrium statistical mechanics. It was originally introduced with the aim to account for phase separation in helium mixtures [30]. Besides various thermodynamic properties, the model has been extended to study the structural phase transitions in many bulk systems. By... 

---